This invention relates to a method and means for increasing the life of carbon aircraft brakes. More particularly, this invention relates to the controlled application of braking pressure to only selected brakes during low speed ground travel.
Modern aircraft which are designed to carry large passenger or cargo payloads are often provided with carbon brakes on each of the wing or body mounted wheels. The nose wheel is typically not braked. Carbon brakes are preferred because of their light weight and performance characteristics and generally comprise a piston housing and parts, a torque plate and a carbon heat sink stack. This stack contains all the friction surfaces which, when compressed, cause the wheel to decrease its speed. The stack comprises a pressure plate, rotor disks, stator disks and backing plate. Carbon composite rotors are connected to the wheel through the rotor drive keys and turn with the wheel. Carbon composite stators, pressure plates and backing plate are connected to the torque tube and do not turn. Braking friction is caused when the rotors are compressed against the stators.
While carbon brakes are preferred for weight and performance reasons over steel brakes, the cost of replacing the stack divided by the number of landing cycles between replacements is much higher than for steel brakes.
Aircraft brake control systems are designed with separate pedal controls for the left and right brakes. When one of the brake pedals is depressed, all the brakes on that side of the aircraft are commanded to apply simultaneously and equally. By applying all brakes equally, the heat energy absorbed by each individual brake is minimized. For steel brakes, brake life is largely determined by the total amount of energy absorbed by each brake and is largely unaffected by the number of brake applications that accumulate that energy. Hence, brake control systems that apply all brakes simultaneously and equally provide economic operation of steel brakes and minimize exposure to overheating of any individual brake. However, direct application of this method to carbon brakes does not extend and may significantly shorten their lives. Accordingly, this invention provides a novel method and means to extend the life of carbon brakes and substantially reduce their operating cost.
In accordance with the invention carbon brake life is significantly extended by decreasing the number of brake applications during each landing cycle. More particularly, brake wear has been found to correlate significantly with the number of brake applications and to not be significantly affected by the energy absorbed during each. By far the largest number of brake applications occur during ordinary taxiing, so in preferred embodiments of this invention, only some of the brakes are applied in response to brake applications under ordinary taxiing conditions. An alternating wheel braking pattern is established to minimize brake wear at each braked wheel and yet to promote even distribution of absorbed energy among all the brakes. This, in turn, prevents overheating of any individual brake. The extended brake-wear system is activated only when aircraft ground speed and brake application pressures are typical of taxi operations. Preferably, aircraft speed and hydraulic pressure are sensed so that brakes at all wheels will be operative in critical braking situations such as landing, parking, or emergency stopping.
The invention will be better understood in terms of the Figures and detailed description which follow.